P2p/configure features

[elenaf9]

Description of change

Remove relay and mdns features that were used in the code to automatically implement certain behaviour, which is bad practice as features are additive (see #210 (comment)).
Instead enable / disable mdns and relay functionality on init via config flags in the StrongholdP2pBuilder, per default both are enabled in the builder.
In the Stronghold client, mdns and relay can be enabled/ disabled via the NetworkConfig when spawning the p2p-network actor. Per default, both are disabled for Stronghold.

Type of change

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)

How the change has been tested

Added new tests for mdns and relay configuration.

Change checklist

Add an x to the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[felsweg-iota]

Looks good to me! Appreciate the change from feature gates to a config based approach.